There are several special edition gear sets in Afterpulse that increase their performance when worn as a set. Since there are three types of gear - head, torso, and legs - sets usually come in groups of three. There is usually a performance increase for wearing two out of three pieces, and then a further bonus when all three pieces are worn at the same time.
At this time, we believe that when a three-piece set is worn, both the two-piece and three-piece set bonuses are in affect.
Gear sets, just like weapons series, tend to be released around holidays - although sets like the Mastodon gear have been in the game at least since its public inception.
The Jester gear set was first introduced on March 31st, 2016. Here is a cool picture of the set.